Sensors for determining and monitoring the limit level of liquid media and flowable solids are sold by the applicant under the marks LIQUIPHANT and SOLIPHANT. Process variables, which besides fill level (limit level detection) can be monitored with vibronic sensors, include especially the process variables, density and viscosity. The applicant sells a sensor suitable for density measurement under the designation, LIQUIPHANT density. Moreover, the applicant is owner of a large number of industrial property rights relating to vibronic sensors for limit-level-, density- and/or viscosity measurement.
Used for driving vibronic sensors is either a piezoelectric bimorph drive or a piezoelectric stack drive. In the case of a bimorph drive, a disc shaped piezoelectric element is connected by force interlocking, e.g. frictional interlocking, with a membrane, to which the oscillatable unit is secured. The disc-shaped element is differently polarized in different segments. In the case of a stack drive, a number of piezoelectric elements are arranged e.g. on a bolt connected among one another and with the oscillatable unit by force interlocking, e.g. frictional interlocking, wherein some piezoelectric elements are excited by an electrical alternating signal to execute oscillations, while the remaining piezoelectric elements register the oscillations of the oscillatable unit and convert such into an alternating electrical response signal.
The oscillatable unit of a vibronic measuring device includes, protruding into the container, preferably two oscillatory rods, which are secured on the outer surface of a membrane facing the medium. Known, however, are also vibronic measuring devices having a single rod. The piezoelectric elements serving as exciter unit are supplied with an alternating voltage signal, whereby the two oscillatory rods of the oscillatable unit are excited to oppositely sensed oscillations directed transversely to the longitudinal axis of the vibronic measuring device. The receiving unit receives the oscillations of the mechanical oscillatable unit and transduces such into an electrical alternating voltage signal. If a change in the oscillation variables appears, e.g. if a frequency change occurs in the oscillations of the oscillatable unit, then this is cause for a corresponding report of the vibronic fill-level measuring device. In the case of application as an overfilling preventer, possible reports include: “Oscillatable unit in contact with the medium”, respectively “limit-level achieved” or “oscillatable unit oscillating freely”, respectively “limit-level not achieved”. For density- and/or viscosity measurement, likewise the alternating voltage response signal is suitably evaluated.
Vibronic measuring devices are sold by Endress+Hauser either in a compact version or in versions with a tube extension and/or a temperature reduction unit. For example, the compact version is so designed that it can be applied at temperatures up to e.g. 100° C. In the case of the compact version, the sensor module—composed of the oscillatable unit and the mechanical, respectively electromechanical, part of the exciter/receiving unit—is connected via a mechanical and electrical/electronic interface directly with the electronics module—i.e. the electrical/electronic part of the exciter/receiving unit and the evaluation unit. In order at temperatures above 100° C. to prevent a degrading or destruction of temperature-sensitive components of the electronics module, a temperature reduction unit is arranged between the sensor module and the electronics module. In such case, the length of the temperature reduction unit depends on the ambient temperature reigning at the location of use of the vibronic measuring device. A tubular extension likewise increases the distance between the sensor module and the electronics module. Moreover, the tubular extension has the job of changing the position of the switching point of the vibronic measuring device in the container. In order that the vibronic measuring device has a comparable behavior in an equal application independently of its length, it is necessary to construct the electronics module differently as a function of the length of the vibronic sensor: resulting from the lengthened electrical lines are coupling capacitances, which superimpose on the alternating voltage signals and influence the behavior of the vibronic measuring device.
An object of the invention is to provide a universally applicable, cost effective, vibronic measuring device.